This article refers to the Force users known as Sith. You may be looking for the Sith Species.
Sith are Force users using the dark side. The word may also refer to the beliefs and code that they follow. Many Orders in Jedi vs Sith use the Sith name, although many are significantly different form the canonical Sith of Star Wars itself. The canonical Sith are described in detail at Sith on Wookieepedia
This page lists Sith Orders and Characters. Other Sith-like Orders may take the name of Dark Jedi and are listed on that page.